How to enable double sided normals or double sided faces for rendering in python API?
You can set Mesh.show_double_sided flag to True for all mesh objects:

import bpy

for obj in bpy.data.objects:
    if obj.type == 'MESH':
        obj.data.show_double_sided = True


Alternatively you can iterate through the mesh data blocks and set the flag:

import bpy

for mesh in bpy.data.meshes:
    mesh.show_double_sided = True